This invention concerns a process for production of fluorinated copolymers by aqueous emulsion polymerization comprising pre-emulsifiying comonomers exhibiting low aqueous solubility. The process is especially useful for producing functionalized fluorinated, especially partially fluorinated, copolymers. It has been found that the copolymers produced herein have certain physical and mechanical properties which are advantageous in electrochemical applications such as batteries, fuel cells, electrolysis cells, ion exchange membranes, sensors, electrochemical capacitors, and modified electrodes, most particularly in lithium batteries and polymer electrolyte membrane fuel cells. Certain compositions of the invention are also useful as strong acid catalysts.
Functionalized polymers are useful in various technologies for generating desirable polymer characteristics. Some applications seek uniform distribution of comonomers for maximal product performance (see Quarderer, G.; Katti, S. S. Polymer Engineering and Science 1993, 33, 564). Other applications, as indicated in the following discussion, are thought to be best served by a combination of properties difficult to obtain from a single substance. For example a polymer suitable for use as an ionically conductive solid polymer electrolyte membrane must exhibit good affinity for suitable solvents and high conductivity while simultaneously exhibiting good mechanical integrity. The polymers produced in the process of the present invention are intended for use in electrochemical devices such as batteries, fuel cells, and electrochemical membrane reactors such as the membrane chlor-alkali process. In these applications, a solvent-swollen ion-exchange membrane separator is desired that can function as a free standing film with sufficient mechanical integrity to separate the electrodes and prevent short circuiting, puncturing, or excessive creep over a wide operating temperature range while under pressure. Polymers with suitable electrochemical properties often fall short in mechanical properties.
To improve mechanical properties of electrochemically preferred polymers, blends with inert fillers or, structural polymers are sometimes employed, as are crosslinking, and the use of fixed porous support membranes and the like. Such approaches often introduce new problems, and have not been found to be fully satisfactory. On the other hand many polymers which exhibit adequate mechanical toughness and strength, lack the needed electrochemical characteristics.
The vinylidene fluoride/lithium perfluorosulfonate ethoxy propyl vinyl ether (VF2/Li-PSEPVE) ionomers and related species disclosed in copending application Ser. No. 09/023,244, which are polymerized, though not hydrolyzed, according to the teachings of the art, address these requirements with only partial success. These polymers, when swollen with quantities of preferred solvents such as dipolar aprotic liquids, exhibit excellent ionic conductivity while retaining desirable mechanical properties and processability. However, the softening temperature of those solvent/ionomer compositions may constrain their application in certain high temperature end uses. The VF2 ionomers formed by the process of the present invention exhibit markedly higher melting or softening temperatures, than do those of similar composition in the art, both in their neat form and in combination with solvents.
It is known in the art that water uptake and ionic conductivity both increase with decreasing equivalent weight in Nafion(copyright) perfluorinated ionomer, the hydrolyzed copolymer of TFE and PSEPVE, available from DuPont.
In actual fact, ionic conductivity exhibits a maximum as a function of decreasing equivalent weight because excessive solvent uptake actually interferes with ionic conductivity. A polymer of low EW of reduced solvent uptake is highly desirable as an approach to achieving the desired mix of properties for electrochemical applications.
WO 9403503 A2 940217 assigned to Imperial Chemical Industries, Ltd. claim polymers of tetrafluoroethylene and sulfonyl fluoride containing perfluorovinyl ethers prepared by emulsion polymerization.
Nakayama, U.S. Pat. No. 5,608,022 discloses an aqueous emulsion copolymerization of tetrafluoroethylene (TFE) and a perfluorocarbon monomer containing a functional group, the latter being in the form of a fine dispersion having an average diameter of 2 micrometers or less, the polymerization being effected in the presence of a water-soluble organic chain transfer agent and a small amount of surfactant.
The present invention is a polymerization process for forming a copolymer comprising:
co-polymerizing in aqueous emulsion one or more monomers selected from the group consisting of tetrafluoroethylene, trifluoroethylene, vinylidene fluoride, vinyl fluoride, ethylene, chorotrifluoroethylene, hexafluoropropylene, hexafluoroisobutylene, perfluoromethyl vinyl ether, and perfluoroethyl vinyl ether with a fluorinated co-monomer having limited water solubility, said comonomer being dispersed in the form of droplets of a size of 10 microns or less, said polymerization process being conducted in the presence of a fluorinated surfactant and a free-radical initiator.
A preferred droplet size is 5 microns or less, and most preferred is 0.5 microns or less. Preferred co-monomers contain sulfonyl fluoride groups and are of the structure:
CF2xe2x95x90CF(OCF2CFR)aOCF2(CFKxe2x80x2)bSO2Fxe2x80x83xe2x80x83I
wherein R and Rxe2x80x2 are independently selected from F, Cl or a perfluoroalkyl group having 1 to 10 carbon atoms optionally substituted with one or more ether oxygens, a=0, 1 or 2, and b=0 to 6.
Another preferred co-monomer class contains carboxylic acid ester groups and is described by the structure:
CF2xe2x95x90CF(OCF2CFR)aOCF2(CFRxe2x80x2)bCO2CH3xe2x80x83xe2x80x83II
wherein R and Rxe2x80x2 are independently selected from F, Cl or a perfluoroalkyl group having 1 to 10 carbon atoms optionally substituted with one or more ether oxygens, a=0 or 2, and b=0 to 6.
Most preferred is (I) wherein a=1, R=CF3, Rxe2x80x2 is F and b=1, known in the art as perfluoro-2-(2fluorosulfonylethoxy)propyl vinyl ether (CF2xe2x95x90CFxe2x80x94Oxe2x80x94CF2xe2x80x94CF(CF3)xe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F is herein abbreviated as PSEPVE).